Wheel dulling sensing circuit

ABSTRACT

A grinding machine which dresses the abrasive wheel only when the wheel dullness reaches a predetermined point as controlled when a determination of stack removal reaches a predetermined amount.

[451 Sept. 25, 1973 lJnited States Patent [191 Robillard et al.

[56] References Cited WHEEL DULLING SENSING CIRCUIT 87 8 mi .67 lOO 1 a mu S WW5 T M: H mm r m n 5 mm B mo T w A wkm T mm SwLZ D E073 T765 Mwww WWW 826 237 578 324 332 m 3 a C yr. no 6 m m mm am RU G M m m Wfv d o EHb s r m n e v n I .J 5 7 Primary Examiner-Harold D. Whitehead Attorney-Norman S. Blodgett [5 7] ABSTRACT A grinding machine which dresses the abrasive wheel [73] Assignee: Cincinnati Milacron-Heald Corp.,

Worcester, Mass.

[22] Filed: Jan. 31, 1972 [21] Appl. No.: 222,058

only when the wheel dullness reaches a predetermined 8 Claims, 3 Drawing Figures point as controlled when a determination of stack removal reaches a predetermined amount.

Field of Search..................... 51/165 R, 165.87, 5l/l65.88, 165.8

PATENTED SEP25|975 SHEET 2 OF 3 FIG2.

BACKGROUND OF THE INVENTION In the operation of grinding machines, it has become the practice to carry out a so-called skip dress only at set times in the grinding of a succession of workpieces. Although these times desirably should be selected to occur Whenthe wheel is dull, nevertheless, it has been the practice to simply dress after a fixed number of workpieces have been finished. Infrequent dressing has the effect of reducing the length of the grinding cycles, which is ad'esirable attribute, in view of the high cost of the capital equipment; it also results in the abrasive'wheel lasting longer. However, in the grinding of high temperature alloys, such as M-50steel, it is very difficult to remove stock unless the wheel is very sharp. Operating with such metals requires frequent dressing, and these conditions have made it very desirable to have some automatic method of detecting the wheel dullness and then to signal the machine for a dress cycle when the dullness reaches a pre-set level.

In the past, various grinding machines have been developed which produce a dressing operation when the abrasive wheel becomes dull. Such a machine is shown in the U.S. Pat. of Lockwood No. 3,327,432 which issued on June 27, 1967 and is provided with an inprocess gage which determines when the workpiece size reaches a finish size. If this size occurs before a switch is contacted in the cross-feed table, this is an indication that the wheel is not cutting as fast as it should and that it needs to be dressed. Another machine that makes use of the dullness of the wheel is the U.S. Pat. to Ware et al. No. 3,535,828 which issured on Oct. 27, 1970; in this particular machine, the dullness control is used during sparkout. The power absorbed by the wheelhead is measured continuously and, if it does not drop below a pre-determined level before a certain time has passed during sparkout, then a dressing operation is set in motion. A still further machine for making use of wheel dullness is shown in the patent application of Hahn et al., Ser. No. 111,608, which was filed on Feb. 1, 1971 and assigned to the assignee of the present invention. This machine is one in which the forces and velocities encountered at the interface between the abrasive wheel and the surface of the workpiece are measured continuously and a measure of dullness taken from these values; the dressing operation takes place, of course, when the dullness reaches a certain value.

None of these methods work particularly well during rough grinding or in the case of tough metals where it is most desirable to renew the surface of the abrasive wheel, for instance, to prevent thermal damage to the workpiece. These and' other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a grinding machine in which the dullness of the abrasive wheel is measured by measuring the rate of stock removal.

Another object of this invention is the provision of a grinding machine incorporating an automatic skip dress" which is operable during a roughing grind.

A further object of the present invention is the provision of a grinding machine using skip dressing which operates particularly well with hard-to-grind, hightemperature metals.

It is another object of the instant invention to provide a machine control in which a dressing operation takes place in response to wheel dullness, the control consisting largely of solid state components which are compact and indestructible.

A still further object of the invention is the provision of a grinding machine using the skip dress principle including control apparatus which is simple in construction, inexpensive to manufacture and which is capable of a long life of useful service with a minimum of maintenance.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

SUMMARY OF THE INVENTION In general, the invention consists of a grinding machine having means for making a determination of stock removal. The said means comprises apparatus for measuring a time interval starting at a point in the movement between the wheelhead and the workpiece and it also includes a device for measuring the progress of the wheel through the workpiece during that time interval. Means is provided for bringing about a dressing operation of the dresser on the wheel when the said determination of stock removal reaches a predetermined amount.

More specifically, the machine includes a switch for the determination of the said fixed point and an inprocess gage for measuring the progress of the wheel through the workpiece during the entire time interval. The timer is inhibited except during a pre-determined portion of the grinding cycle. A memory device is provided for holding an indication of workpiece size at the start of the time interval for the purpose of comparing it with the size at the end of the interval.

BRIEF DESCRIPTION OF THE DRAWINGS The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a perspective view of a grinding machine embodying the principles of the Present invention,

FIG. 2 is a schematic view of a portion of the grinding machine taken on the line IIII of FIG. 1, and

FIG. 3 is an electrical schematic view of a portion of the grinding machine controls.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, which best shows the general features of the invention, it can be seen that the grinding machine, indicated generally by the reference numeral 10, is of the type shown and described in the U.S. Pat. of Hohler et al. No. 3,568,374 which issued Mar. 9, 1971. It is provided with a lower base A on which is mounted a workhead 14 and a wheelhead 25 having a drive motor 50. Around the front of the base extends a splash guard B which is readily removable. Extending upwardly from the rear of the base A is a superstructure C having two arms D which extend forwardly from the end of the base. Mounted between the arms is a control cabinet E. At one end of the machine is located a coolant tank F receiving coolant returned from the machine through a pipe G.

FIG. 2 is approximately a horizontal view through the machine taken on the line IIII of FIG. 1. It shows somewhat schematically the base A on which is mounted the workhead 14. This workhead is constructed to support and rotate a workpiece 13 which has a surface of revolution 12 to be generated by the use of the abrasive process. The workhead is mounted on a table 15 which is mounted on the base A by means of ways 16 and 17 for movement in the direction of the axis of the surface of revolution 12 under the impetus of a hydraulic cylinder 18. This cylinder is connected by conduits to suitable valving and source of pressure fluid in the usual way to produce reciprocation during the grinding cycle. The workhead 14 is connected to a suitable motor 20 driven by electrical power supplied through suitable electrical lines. Mounted on the table 15 is a diamond dressing apparatus 11.

Also mounted on the base A is the wheelhead 1'0- tatably carrying a spindle 26 which has mounted at its outboard end an abrasive wheel 27. The wheelhead 25 is connected to the electric motor 50 for driving the spindle and abrasive wheel, the motor being supplied with electrical power in the usual way. The wheelhead is mounted on a table 40 which is slidable on ways 30 and 31 transversely of the axis of the spindle 26 and of the axis of the surface of revolution 12. The motion is brought about by a main feed cylinder 32 which is connected by suitable conduits to the usual valves and source of pressure fluid. Slidably carried on top of the base under the table 40 is a compensation slide 35 which is capable of being moved back and forth transversely of the axis of the spindle 26 by means ofa stepping motor 36 operated by electrical pulses. The table 40 is provided with a horn 39 which, on occasion, acts as a stop in combination with a notch 41 in the compensation slide 35. Pressing against the front of the slide 35 is a contact shaft 46 threadedly engaged with a nut fastened to the underside of the table 40. The shaft 46 is rotated on occasion by means ofa feed stepping motor which receives pulses in a manner similar to the motor 36.

Mounted on the base A is a control 28 which receives a signal from an in-process gage 24. This gage 24 is of the well-known type which gages the size of the workpiece during the grinding process and produces a voltage proportional at any time to the size of the workpiece. The gage is similar to the one shown and described in the patent application of Uhtenwoldt et al., Ser. No. 104,859 in which the signal from the pneumatic gage is converted to electrical voltage values.

Referring to FIG. 3, it can be seen that the gage 24 is a pneumatic gage receiving air from a conduit 51 having a branch leading to an enclosed diaphragm 52. The diaphragm is connected by a stem 53 to the center tap 54 of a potentiometer 55. One end of the potentiometer is connected to ground and the other end to a source of-l 2-volt electrical power. The lead 56 is connected from the center tap 54 to the circuit at a point 57. A switch 58 is connected to be opened when the cross-feed table 40 reaches a certain position (see FIG. 2). It is connected on one side of the +12-volt line 59 and on the other side to one side of the coil of a relay lRR. The coil of the relay lRR is connected on its other side to a line 60 forming part ofa timer. The l2 volt point 61 is also connected into the circuit.

Also located in the circuit is a normally open contactor lRR-l (operated by the relay coil lRR) as well as a relay coil 2RR which operates a normally open contactor 2RR-l. A relay coil 3RR operates a normally open contactor 3RR-1 which, when closed, serves to set in motion in the usual way the dressing operation. The upper portion of the electrical schematic shown in FIG. 3 includes timing apparatus, while the lower portion is used for the determination of stock removal. The side of the normally open contactor 2RR-1 opposite the point 57 is connected through a resistor R8 to one side of a capacitor C6. In the preferred embodiment, this capacitor has a value of 0.1 microfarad and is a special type known as a low leakage capacitor which is capable of maintaining its charge for long periods of time. The other side of the normally open contactor 2RR-1 is also connected to one of the inputs of a field effect transistor type operational amplifier 62 which has a high input impedance. The output of this is connected to one of the inputs of an inverting amplifier 63 which, in turn, is connected to an amplifier 64. The output of the amplifier 64 is connected to an input of a comparator amplifier 65 whose other input is connected to a variable resistor R17, which is connected on one end to ground and on the other end to the +12 volt line 59. The center tap of this variable resistor is connected to one of the inputs of the amplifier 65. Also included in this circuit is a 2N2l93 transistor 04.

The timer associated with the line 60 includes a 8.2 microfarad capacitor C-1 and three serially connected transistors including a 2Nl67lB transistor Q1, a 2N2l93 transistor Q2, and a 2N2l93 transistor Q3. Logic gates 66, 67, 68, and 69 are associated with the timing portion of the circuit. One-shot pulse generators 70 and 71 are also associated with the circuit.

The operation of the apparatus will now be readilyv understood in view of the above description. The workpiece 13 is mounted on the workhead l4 and driven by the motor 20. An abrasive wheel 27 on its spindle 26 is driven by the wheelhead 25 through the medium of the motor 50. The cylinder 18 is operated to move the table 15 longitudinally to introduce the abrasive wheel 27 into the workpiece 13 for finishing the surface 12. A cylinder 32 moves the table 40 in a cross-feed direction to engage the abrasive wheel 27 with the surface 12 of the workpiece. The gage 24 continuously measures the size of the hole represented by the surface 12 and transmits this information to the control 25.

Referring to FIG. 3, the operation of the gage 24 causes the diaphragm 52 to move the center tap 54 of the potentiometer 55, so that a voltage appears on the line 56, this voltage becoming less as the size of the bore increases. So long as the contact 58 is closed, the circuit is inhibited by the energization of the relay lRR, so that the normally open contact lRR-l will be closed. This will hold the center diode D1 at zero potential and disable the timing circuits. The contactor lRR-l will also hold a logic 0" on the pin of the buffer gate 69, thus holding the relay 3RR de-energized. The contactor lRR-l when closed will also connect the zenith diode D-4 at zero potential causing the coil 2RR to be energized. This means that the contactor 2RR-l (adjacent the gage input signal point 57) will be closed causing the input voltage from the point 57 to feed into the reference amplifier/memory circuit consisting of the amplifiers 62 and 63.

The loop gain of this circuit is adjusted by means of a trim potentiometer R10, so that the voltage at the pin 106 of the amplifier 64 is 0 volts. This voltage when compared to the positive dullness reference voltage from the adjustable resistance R17 will cause the output of the transistor 65 to be negative and the transistor Q4 will be cut off causing a logic 1 (+12 volts) to be present at the input pin 107 of the buffer gate 67. The other gate input (pin 116) is connected to the output pin 114 of the one-shot pulse generator 71 which is at a logic 0" at this time. This will disqualify the gate 67 and its output pin 104 will be at a logic 1.

To start the circuit in operation, the inhibit terminal is de-energized by the opening of the switch 58. This will de-energize the relay 2RR which sets the memory reference; the input voltage to the resistor R12 from the transistor 63 will be fixed. The input signal voltage from the gage 24 and the potentiometer 55 is still connected to a resistor R1 1 and it is free to vary as the signal voltage changes.

Also, at this time, the opening of the contactor lRR-l ungrounds the zener diode D-1 and allows the measuring timer to start timing. When the capacitor C-1 of the measuring timer charges up to the peak voltage (firing point) of the unijunction transistor Q1, it discharges through the resistor R6 producing a positive pulse at the base of the transistor Q2. This positive pulse is amplified by transistors Q2 and Q3 and appears at the inputs. of the two l-shot pulse generators 70 and 71 causing them to fire.

The pulse duration for the upper pulse generator 70 is much longer than that of the bottom generator 71 because of the larger value of the external timing capacitor C4 that is used. The +12 volt(logic 1) will be fed from the output pin 114 of the bottom generator 71 to the input pin 116 of the buffer gate 67. The other input pin 107 of the gate 67 is connected to the output of the transistor Q4. This voltage would either be +12 volts or 0 volts (either logic l or logic 0), depending upon the amount of voltage change that occurred in the input signal during the measuring time. If this voltage was greater than the pre-set reference set by the variable resistor R17, the output of the comparator amplifier 65 will be positive and the transistor Q4 will be ON causing a logic 0 at its output and at the pm 107 of the butter gate 67. Such a situation will disqualify the gate and the relay 3RR will remain de-energized. If the voltage change was less than the pre-set reference, this is an indication that the wheel is dull and the output side of the comparator amplifier 65 will be negative and the transistor Q4 will be ofi causing a logic 1" at its output and at the pin 107 of the buffer gate 67; this, along with the logic 1 pulse at the pin 116 arriving from the generator 71 will qualify the gate and a logic 0 pulse will appear at its output terminal pin 104 and at the pin 110 of the buffer gate 68. This last gate is interconnected with another buffer gate 69 to form a set-reset type of flip-flop or memory circuit. The logic .0" pulse will set the circuit on and cause the relay 3RR to be energized which will signal the grinding machine for a dress interrupt cycle.

A logic 0" pulse will appear at the output pin 111 of the lower generator 71 and at the input pin 103 of the buffer gate 66. This will disqualify this gate until the lower generator 71 (short duration pulse) has completed its pulse. When this happens, the pin 111 goes back to a logic l and with the long duration logic 1 6 pulse at the output pin 102 of the generator connected to the input pin of the gate 66, the gate will be qualified and its output pin 101 will go to a logic 0.

This will energize the relay ZRR and re-set the memory reference and comparator circuit for the start of a new timing cycle.

A long duration logic 0 pulse also occurs at the output pin 105 of the generator 70. This pulse is connected to the diode D2 and causes the timing capacitor C1 to fully discharge. When this long duration pulse is ended, if the relay 3RR is not energized, the timing cycle will again be started and the change in gage signal will again be sampled for another fixed duration of time.

Ingeneral, it can be seen that by use of the present invention it is possible to check the amount of stock that is removed from the workpiece during a pre-set interval and compare it to a predetermined reference. If the stock removed is greater than the reference, the wheel is still sharp'and the control circuit continues to cycle. When a reading shows that the stock removed was less than the reference, a relay in the controller is energized. A contact from this relay is used in the machine control circuit to initiate a wheel dressing cycle. This circuit requires an input signal from the in-process gage and the signal is a negative-going D.C. voltage. The signal from the machine control circuit is used to inhibit this circuit except for that part of the grinding cycle when the circuit should be active. This is usually during the rough grind. The inhibiting signal is of the on-off type. The output from the dullness sensing circuit is a reed relay contact that closes when the wheel is dulled to a pre-set level. This contact is connected to the machine controls such that a dress-interrupt is initiated when the contact closes.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A grinding machine, comprising a. a base,

b a workhead having a workpiece-holding means mounted on the base,

c a wheelhead having a rotatable abrasive wheel mounted on the base,

d a dresser operative to dress the wheel on occasion,

e means for bringing about relative movement longitudinally and transversely of the wheel axis between the wheelhead and the workhead to produce grinding cycles on successive workpieces,

1' control means for making a determination of stock removal, the said means comprising apparatus for measuring a time interval starting at a point in the movement between the wheelhead and the workpiece and a device for measuring the progress of the wheel through the workpiece, and

g means for bringing about a dressing operation of the dresser on the wheel when the said determination of stock removal reaches a predetermined amount.

2. A grinding machine as recited in claim 1, wherein the said control means includes a switch for determination of the said point.

3. A grinding machine as recited in claim 1, wherein the said device measures the progress of the wheel at all times and includes a memory device for retaining the value of the voltage at the start of the time interval.

7. A grinding machine is recited in claim 6, wherein the said voltage at the start is subtracted from the voltage at the end of the time interval and this difference is compared with a predetermined voltage that is set to represent a desirable amount of stock removal.

8. A grinding machine as recited in claim 7, wherein the memory device is a low-leakage capacitor. 

1. A grinding machine, comprising a. a base, b a workhead having a workpiece-holding means mounted on the base, c a wheelhead having a rotatable abrasive wheel mounted on the base, d a dresser operative to dress the wheel on occasion, e means for bringing about relative movement longitudinally and transversely of the wheel axis between the wheelhead and the workhead to produce grinding cycles on successive workpieces, f control means for making a determination of stock removal, the said means comprising apparatus for measuring a time interval starting at a point in the movement between the wheelhead and the workpiece and a device for measuring the progress of the wheel through the workpiece, and g means for bringing about a dressing operation of the dresser on the wheel when the said determination of stock removal reaches a predetermined amount.
 2. A grinding machine as recited in claim 1, wherein the said control means includes a switch for determination of the said point.
 3. A grinding machine as recited in claim 1, wherein the said device measures the progress of the wheel through the workpiece during the entire time interval.
 4. A grinding machine as recited in claim 1, wherein the said apparatus is a timer whose operation is inhibited except during a predetermined portion of the grinding cycle.
 5. A grinding machine as recited in claim 4, wherein the apparatus consists of a timer that operates sequentially through a portion of the grinding cycle.
 6. A grinding machine as recited in claim 1, wherein the said device includes a in-process gage which generates a voltage proportional to the size of the workpiece at all times and includes a memory device for retaining the value of the voltage at the start of the time interval.
 7. A grinding machine is recited in claim 6, wherein the said voltage at the start is subtracted from the voltage at the end of the time interval and this difference is compared with a predetermined voltage that is set to represent a desirable amount of stock removal.
 8. A grinding machine as recited in claim 7, wherein the memory device is a low-leakage capacitor. 